1. Field of the Invention
The present invention relates to a transparent, colorless glass or glass-ceramic pane;, which is exposed to a high heat load in operation and which has a full-surface or partially-covering visually dense, high-temperature-stable coating in the form of an organic/inorganic network structure provided with a color-imparting pigment.
The invention also relates to a method of making this sort of coated glass or glass-ceramic panel.
The invention further relates to a cooking unit with a coated glass or glass-ceramic panel of this sort, which provides a cooking surface.
2. Related Art
The term “glass or glass-ceramic panel” in the context of the present invention means not only flat, planar panels, but also angular, beveled and bent panels as well as curved panels. The panels can be rectangular or round or circular and also they can have other shapes. The glass panels according to the invention can only be made of glass having a very low thermal expansion coefficient and sufficiently “hard”, for example of pre-stressed borosilicate glass.
The technical problem to be solved with the invention will be explained in the following with the aid of a typical application for a flat planar glass-ceramic panel providing the cooking surface of a modern cooking range, but the present invention should not be considered to be limited to that application. This technical problem also exists with glass panels providing cooking surfaces and in other applications, such as with glass or glass-ceramic fireplace window panes, glass or glass-ceramic interior windows of oven doors or glass or glass-ceramic light covers, “mutatis mutandis” in similar ways.
Cooking ranges with a glass-ceramic panel providing a cooking surface have been marketed in the art. The glass-ceramic material is transparent in the optical region of the spectrum. Thus radiation, for example from kitchen lighting, can pass through it from above, or from below, for example, from heating elements in radiatively heated cooking zones, so that heating elements, cables and other structural elements, which should be hidden under the glass-ceramic cooking surface, can be seen.
The observation of these structural elements is however considered to be troublesome for the user. Thus the glass-ceramic panels for the cooking surfaces, typically as can be seen from EP 0 220 333, are colored with color-imparting ions to reduce the transmission in the visible range, so that the operating parts of the cooking range below the glass-ceramic panel are practically invisible from above. These cooking surfaces are thus practically not transparent, i.e. opaque, for radiation in the visible range and appear to be black or for example dark red-violet or orange/brown, when viewed in transmission according to the color-imparting ions that are used. These opaque glass-ceramic materials can only be integrated in limited display forms, for example to indicate residual heat, which is a fundamental disadvantage.
A color-imparting visually dense coating applied to the underside of the translucent glass-ceramic panel providing the cooking surface in the area for introducing displays is one known solution to the problem of optical non-transparency of the glass-ceramic or glass panel. Thus JP H 7-17409 and JP 51-89517 discloses a glass-ceramic panel providing a cooking surface, which comprises a transparent, colorless glass-ceramic material with a high-temperature-resistant paint printed on its underside. This paint layer is built up so that it is made non-transparent, i.e. it replaces the otherwise conventional color, so that the cooking surface appears to be black when viewed.
In order to provide the necessary degree of optical or visual density, the paint must be applied in a comparatively thick layer. However the different thermal expansion coefficients of the glass-ceramic panel and the paint layer create the problem that cracks can appear in the paint layer or in the coated glass-ceramic surface. Furthermore the paint layer can even partially flake or peal off because of the cracking.
Transparent glass-ceramic cooking surfaces with a visually dense coating on their underside are also disclosed in DE 100 14 373 A1, which is equivalent to WO 01/72087 A1, and DE 200 19 210 U1. The underside coating provides a decoration at the same time in the case of the first reference, DE 100 14 373. A1. In contrast the underside coating is a single uniform color and the cooking surface on the top side of the panel provides a full-surface decorative coating in the case of the second reference, DE 200 19 210 U1.
Especially lustrous paints, paints based on organic materials, paints based on glass flux, especially with borosilicate glass flux and titanium or cerium oxide as pigments, colored or pigmented sol-gel coatings, which contain conventional inorganic pigments, lustrous pigments, metal effect pigments, pearlesence imparting pigments or mixtures of these pigments, are useful as paints for this coating applied to the underside of a glass or glass-ceramic panel.
The application of the paint to the underside of the glass or glass-ceramic panel occurs by printing. The glass-ceramic panel can be knobbed and also smooth on both sides. The knobbed panels are generally considerably less mechanically sensitive in comparison to the panels that are smooth on both sides. However more problems are generally encountered printing the knobbed panels.
Generally the colored underside coatings described in the previous references are not visually dense when applied with a single printing. The optical density of the coating can be increased by applied multiple layers. That is a comparatively thick underside coating is necessary to provide the required optical density.
This is of course not true for paint based on organic material, because these paints do not reduce the strength of the glass-ceramic cooking surface or only slightly reduce it. The disadvantage of these organic paints however is that they have only a limited resistance to high temperatures because of their organic components and are irreversibly discolored. Typical ingredients, such as silicone, polyester or resins, decompose above 400° C. However the cooking zone underside typically reaches temperatures up to 600° C. in continuous operation. Temperatures of as much as 800° C. can be reached for a short time. The above-mentioned organic paints decompose under heat load, whereby decomposition products are released and the coating no longer adheres after a certain time.
Besides printing a coating on the underside of the glass-ceramic panel, it is also known from DE 101 22 718 A1 to apply a plasma-sprayed coating to the underside of the glass-ceramic panel, which further increases the opacity of the glass-ceramic panel for visible light (also provides a bulk-coloring) and/or provides protection from the propagation of scattered light in halogen-heated cooking systems. Generally there are no disclosures regarding the strength of the resulting coated cooking panels in this reference. Generally this process has an especially critical variable, since the strength of the glass-ceramic panel can be considerably reduced by bombardment of the glass-ceramic surface with hot particles. Also the color choice is considerably limited in this process. Of course colored pigments can be added to the actual coating matrix, but generally the spraying of the colored pigments in this process is generally not possible because of their high melting points. Also only a certain tinting or shading of the coating is attainable with this process.
According to the state of the art several different coating steps are required during coating of a glass-ceramic panel to obtain various optical impressions, such as different colors, mixed colors, metallic colors, black. A combination of different coating methods (screen printing, vacuum coating methods) is often required, especially so that the underside coating provides a metallic impression. Sometimes the desired impression can only occur by a precise harmonization of a full-surface top side coating and underside coating. Likewise the production of the desired optical density is not possible in a single coating step and often only by combination of top side and underside coatings.
This is also true for the above-described sol-gel coatings, which are colored with the most different pigments and/or mixtures of them. Without further steps regarding composition of the sol-gel solution, the production of the colors by the pigments and application of the sol-gel colored coating to the surface of the glass-ceramic panel, the above-described disadvantages cannot be avoided.
The state of the art in the area of sol-gel coatings provides no suggestion for suitable solutions of the foregoing problems. WO 96/29447 describes pigmented sol-gel coatings for application to glass, ceramic material or metal at high temperatures. Different sols are combined with various colored pigments, in order to obtain functional coatings, e.g. against abrasion by utensils. There is no sol, which would be compatible with all particles that are used, also with no mixtures of the particles used. Also the coating of glass-ceramic is not described. The largest coating thickness obtained by a single layer application was 6 μm. In, order to obtain greater coating thickness, e.g. 10 μm, multiple coating applications are necessary. The sol-gel coating is then burned in at temperatures up to 1000° C. and because of that melted on the substrate surface, which would lead to a reduction of the strength of the coated glass-ceramic panel or glass panel.